Power distribution in a portable device

ABSTRACT

A power distribution system is provided for distributing power in a portable device being coupable to a charging system and including a main battery power and a bridge battery power. The charging system is coupled to a power bus through a diode. The main battery power system and the bridge battery system are also both coupled to the power bus through diodes. The predetermined voltage level is set with respect to the charging system, the main battery and the bridge battery, such that if all three are coupled to the bus, the recharging system both powers the unit and recharges the main battery. If the recharging system is removed, the main battery power system powers the portable device, and if the main battery power system is removed the bridge battery will provide power to the portable device. The distribution system also includes a monitoring system for measuring the predetermined charge level on the battery and disabling a FET switch, so that the main battery stops charging to protect the main battery against overcharging. A monitoring system is also provided for monitoring the voltage level of the main battery utilizing a comparator system.

TECHNICAL FIELD

[0001] The present invention relates to a portable devices and moreparticularly to a low cost power distribution system and method for aportable device that is powered by a recharger unit or alternatively arechargeable battery. The portable device includes a bridge battery forpowering the portable device in the event the rechargeable battery isremoved.

BACKGROUND OF THE INVENTION

[0002] Portable electronic devices (e.g. notebook computers, cellulartelephone, cordless telephones mobile data terminals, radio frequencyportable communication devices, etc.) typically include a rechargeablebattery that is charged by a recharging unit plugged into an AC powersource, such as that found in conventional 115 VAC lines. The rechargingunit powers the portable device, while simultaneously charging therechargeable battery. The portable device switches over to battery powerupon removal of the portable device from the charging unit or the powersource. Most intelligent portable devices employ a bridge battery thatis utilized during the removable of the main rechargeable battery whenit powers the portable device. The main function of the bridge batteryis to insure that data stored in a Random Access Memory (RAM) device isnot lost during swapping of the rechargeable battery. The bridge batteryis usually a much smaller battery because during a battery swap, theportable device normally will enter a low current or suspend mode, sothat most current drawn from the bridge battery is used to refresh theRAM device, until a new main battery is installed into the device.

[0003] Many portable devices employ complicated switching or logiccircuitry when changing from the rechargeable device power to the mainbattery power, and from the main battery power to the bridge batterypower. The switching or logic circuitry takes up valuable space on acircuit board that could be utilized to perform the functions of theportable device. In some cases, additional circuit boards are necessaryto house the switching or logic circuitry. This increases the size andcost of the portable device. Furthermore, some of these portable devicesemploy expensive power management and supervisory circuitry to cause thefunctional components in the portable device to enter different modes,depending on the type of unit powering the portable device or thecurrent status of the unit powering the portable device. The additionalpower management and supervisory circuitry adds to the size, the costand also to the complexity of the portable device.

[0004] Accordingly, there is a strong need in the art for a systemand/or method that can provide power distribution for a portableelectronic device at low cost, while reducing space utilized inemploying such a system.

SUMMARY OF THE INVENTION

[0005] According to a preferred embodiment of the present invention, apower distribution system is provided for distributing power in aportable device being coupable to a charging system and including a mainbattery power system and a bridge battery power system. The chargesystem is coupled to a power bus through a diode. The main battery powersystem and the bridge battery system are also both coupled to the powerbus through diodes. A predetermined voltage level is set with respect tothe charging system, the main battery and the bridge battery such thatif all three are coupled to the bus, the recharging system both powersthe unit and recharges the main battery. If the recharging system isremoved, the main battery power system powers the portable device, andif the main battery power system is removed, the bridge battery willprovide power to the portable device. The distribution system alsoincludes a monitoring system for measuring a predetermined charge levelon the battery and disabling a FET switch, so that the main batterystops charging to protect the main battery against overcharging. Amonitoring system is also provided for monitoring the voltage level ofthe main battery utilizing a comparator system. Accordingly, the presentinvention provides for a very low cost power distribution system forportable devices.

[0006] Thus, according to one aspect of the present invention, a powerdistribution system is provided that provides power to functional andpower circuitry on a portable device. The system includes a main batterysystem coupled to a power bus through a first switch system and acharging system coupled to the power bus through a second switch system.The charging system provides power to the power bus through the secondswitch system while the charging system is coupled to the second switchsystem and the main battery system provides power to the power busthrough the first switch system upon removal of the charging system.

[0007] In accordance with another aspect of the present invention apower distribution system is provided that provides power to functionaland power circuitry on a portable device. The system includes a mainbattery system coupled to a power bus through a main battery diode and abridge battery system coupled to the power bus through a bridge batterydiode. The voltage level of the bridge battery is lower than the voltagelevel of the main battery and removal of the main battery system causesthe second switch system to allow the bridge battery system to providepower to the power bus.

[0008] In accordance with yet another aspect of the present invention, amethod is provided for power distribution in a portable device includinga power bus coupled to a device power and functional circuitry. Themethod includes the steps of providing a main battery system coupled toa power bus through a main battery diode, providing a charging systemcoupled to the power bus through a charger diode and providing a bridgebattery system coupled to the power bus through a bridge battery diodewherein the charging system has a voltage level above the main batteryvoltage level and the main battery has a voltage level above the bridgebattery, such that the charger powers the power bus if the charger ispresent, the main battery powers the power bus if the charger is notpresent and the bridge battery powers the bus if the charger and themain battery are not present.

[0009] In accordance with another aspect of the present invention, apower distribution system is provided that provides power to functionaland power circuitry on a portable device. The system includes means forproviding main battery power. The means for providing main battery powerincludes means for coupling the means for providing main battery powerto a power bus. The system also includes means for recharging the meansfor providing main battery power. The means for recharging the means forproviding main battery power includes means for coupling the means forrecharging to the power bus. The means for charging having a voltagelevel that is at a predetermined level above a voltage level of themeans for providing main battery power causing the means for charging tosimultaneously provide power to the power bus and recharge the means forproviding main battery power.

[0010] In accordance with yet another aspect of the present invention, apower distribution system for a providing power to functional and powercircuitry on a portable device is provided. The system includes a mainbattery system coupled to a power bus through a main battery diode, acharging system coupled to the power bus through a charger diode and abridge battery system coupled to the power bus through a bridge batterydiode. The charging system has a voltage level that is at apredetermined level above a voltage level of the main battery system andthe voltage level of the bridge battery system is lower than the voltagelevel of the main battery system. If the charging system is present, thecharger diode is forward biased and the charging system provides powerto the power bus. If the charging system is not present, the mainbattery diode becomes forward biased and the main battery systemprovides power to the power bus. If the charging system and the mainbattery system are not present, the bridge battery diode becomes forwardbiased and the bridge battery system provides power to the power bus.

[0011] To the accomplishment of the foregoing and related ends, theinvention then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the invention. These embodiments are indicative, however,of but a few of the various ways in which the principles of theinvention may be employed and the present invention is intended toinclude all such embodiments and their equivalents. Other objects,advantages and novel features of the invention will become apparent fromthe following detailed description of the invention when considered inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1a illustrates a block diagram of a portable device coupledto a power source in accordance with one aspect of the presentinvention;

[0013]FIG. 1b illustrates a block diagram of a portable device coupledto a power source in accordance with another aspect of the presentinvention;

[0014]FIG. 2 illustrates a block diagram of a portable device coupled toa charger in accordance with another aspect of the present invention;

[0015]FIG. 3 illustrates a block diagram of a portable device includinga low battery voltage monitor in accordance with the present invention;

[0016]FIG. 4 illustrates a flow diagram for providing power to theportable device and the rechargeable main battery in accordance with thepresent invention;

[0017]FIG. 5 illustrates a flow diagram for providing bridge batterypower to the portable device in the event of removal of the main batterypower in accordance with the present invention; and

[0018]FIG. 6 illustrates a flow diagram for providing a low batterypower warning signal to the portable device in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will now be described with respect to theaccompanying drawings in which like numbered elements represent likeparts.

[0020] Referring initially to FIG. 1a, a portable device 10 is providedwith a charger 40 coupled to a power source 30. The source 30 ispreferably AC power and operates at an AC power line frequency of 60 Hz.However, it is appreciated that in other countries different linefrequencies (e.g., 50 Hz) may be employed. The charger 40 provides powerto a power bus 90 for use with powering device power and functionalcircuitry 55 and recharging a main battery 65. The charger 30 is coupledto the power bus 90 through a first switch system 50. A bridge battery95 is provided coupled to the power bus 90 through a second switchsystem 60. The main battery 65 is coupled to the power bus 90 through asecond switch system 70. The charging system 40 powers the portabledevice 10 while the power source 30 is connected to the charger 40. Thefirst switch system 50 provides power to the power bus 50, when thepower source 30 is connected. After the power source 30 is removed, thefirst switch system 50 disables and the third switch system 70 allowspower to be provided to the power bus 90 from the main battery 65. Ifthe main battery is disabled, the third switch system 60 disables andthe second switch system 60 allows power to be provided to the power bus90 from the bridge battery 70.

[0021] In a preferred aspect of the invention, the first switch system50 and the second system 60 are replaced with diodes and the thirdswitch system is replaced with a diode and FET combination. Referringnow to FIG. 1b, the portable device 10 is provided with the charger 40coupled to the power source 30. The charger 40 provides power to thepower bus 90 for use with powering device power and functional circuitry55 and recharging the main battery 65. The charger 30 is coupled to thepower bus 90 through a charger diode 51. The charger diode 51 beginsconducting when the power source 30 is connected to the charger 40.Preferably, the charger is a lithium ion charger. However, it should beappreciated that the charger could be other types of chargers besidelithium, depending on the type of battery being recharged. The mainbattery 65 is coupled to the power bus 90 through a main battery diode75. The main battery diode 75 is connected to a Field Effect Transistor(FET) switch 71 in a parallel relationship. The opening and closing ofthe FET switch 71 is controlled by a charge status monitor 45. Thecharge status monitor 45 could include a timer circuit that beginscounting when the charger 40 begins operation by being connected to thepower source 30. The timing circuit could include a processor (notshown), such as Intel's 8 bit microcontroller, the 8031, 8051 or 8052.Furthermore, the manner in programming the processor to operate theinvention according to the description herein will be apparent to thoseskilled in the art of computer programming. The FET switch 71 is closedupon detection of the charger 40 allowing the charger 40 to charge themain battery 65. After a predetermined amount of time, the charge statusmonitor 45 changes the state of the FET switch 71, so that the FETswitch 71 enters an open state and stops charging the main battery 65.

[0022] The removal of power source 30 causes the charger diode 51 tostop conducting. This causes the charger 40 to stop charging the mainbattery 65 and to stop providing power to the device power andfunctional circuitry 55. Turning off the lithium charger 40 results in avoltage drop on the power bus 90. The positive voltage drop between themain battery 65 and the power bus 90 causes the main battery diode 75 tobegin conducting, such that the device power and functional circuitry 55are powered by the main battery 65. It is well known in the art that adiode becomes forward biased when a voltage drop of about 0.6-1.0 volts,depending on the type of diode, is provided from a cathode side to ananode side causing the diode to conduct and resemble a short circuit.Furthermore, the diode resembles an open circuit if the voltage drop isless than 0.6-1.0 volts or is a positive voltage drop from the anode tothe cathode.

[0023] The bridge battery 95 is provided coupled to the power bus 90through a bridge battery diode 61. The bridge battery 95 is provided tosupply power to the portable device 10 in the event that the mainbattery 65 fails or is being replaced. The bridge battery 95 is a muchsmaller battery than the main battery 65 and functions to providecurrent mostly to a RAM device within the device power and functionalcircuitry 55. The other circuitry utilized in the portable device 10operates in the suspend or low current mode, so that only a small amountof current is necessary to continue operation of the portable device 10.For example, the bridge battery 95 typically may provide 1-1.2 millampsof current to the portable device 10 when it operates in the suspendmode, while the main battery 65 may be supplying 1-1.2 amps duringnormal operation of the portable device 10.

[0024] The main battery 65 is preferably at a voltage range of between3-4.2 volts with a nominal voltage of 3.6 volts, while the voltage rangeof the bridge battery is preferably at 2-3.2 volts with a nominalvoltage of 2.4 volts. In the event that the main battery 65 is removedor malfunctions, the main battery diode 75 stops conducting. Thisresults in a voltage drop on the power bus 90. The positive voltage dropbetween the bridge battery 65 and the power bus 90 causes the bridgebattery diode 61 to begin conducting, such that the device power andfunctional circuitry 55 are powered by the bridge battery 65. If themain battery 65 is replaced, the voltage level on the bus rises abovethe voltage of the bridge battery and causes the bridge battery diode 61to stop conducting. Thus, by utilizing diode coupling to the power bus90, a power distribution system is provided that is inexpensive, takesup very little room on a circuit board, and is simple to fabricate.

[0025] It should be appreciated that the lithium charger 40 could be aseparate unit from the portable device 10. For example, FIG. 2illustrates a system where the charger 40 is a separate device coupledto the portable device 10 via a charge status circuit 80. In thisembodiment of the invention, the charge status circuit 80 monitors thecharge current of the battery utilizing conventional current sensingtechniques. If the charge current drops to a certain level indicatingthat the main battery 65 has become fully charged, the charge status 80will alternate the state of FET switch 71 causing the switch to open.The charge status circuit 80 could monitor the temperature of the mainbattery 65 to determine if it has reached full charge capability. Itshould be noted that the charger 40 could take the form of aconventional cradle device that the portable device 10 rests in, as itrecharges.

[0026] Referring to FIG. 3, the portable device 10 could include a lowmain battery power warning system. The portable device 10 could utilizea comparator 85 to monitor the voltage level of the power bus 90. In theembodiment shown in FIG. 3, the positive terminal of the comparator isconnected to the power bus 90, while the negative terminal of thecomparator 85 is connected to a reference voltage labeled as V_(BatLow).The output of the comparator 85 labeled as “Low Main Battery” changesstate when the voltage level on the power bus 90 drops below thereference signal. The output signal can be coupled to the unit power andfunctional circuitry 55 to perform a power down routine on the portabledevice 10 or cause the portable device to enter the suspend mode. Theoutput signal can also be coupled to a display or a Light Emitting Diode(LED) to provide a visual warning to the user.

[0027]FIG. 4 illustrates the steps taken by the present invention toensure that the charger 40 provides power to the unit power andfunctional circuitry 55, and also performs the function of rechargingthe main battery 65 when the charger 40 is operably coupled to the powerbus 90 via the diode 51. Beginning in step 120, the potable device 10 iscoupled to the charger 40 and the charger 40 is powered up. This may beaccomplished by coupling the portable device 10 to the power source 30,as shown in FIG. 1, or by coupling the portable device 10 to the charger40, as shown in FIGS. 2-3. This depends on whether or not the charger 40is a separate unit from the portable device 10 or is part of portabledevice 10. In step 130, the charger diode 51 begins conducting becausethe voltage level produced by the charger is above the voltage level ofthe main battery 65 and the bridge battery 95 causing the charger diode51 to become forward biased and begin conducting. In step 140, thecharger provides power to the power bus 60 and runs the portable device10. In step 160 the FET switch 71 is turned on, which allows the charger40 via the power bus 90 to provide current to the main battery 65, thusrecharging the main battery 65. The charge status monitor 80 monitorsthe charge provided to the main battery 65 from the charger 40. Aspreviously discussed, the charge status monitor 80 could monitor thecharge by utilizing a timer device or monitor the current draw utilizingconventional current measurement techniques. The charge status monitor80 determines if the main battery 65 has met a predetermined chargelevel. If no, the charger 40 continues charging the main battery 65. Ifyes, the charge status monitor turns the FET switch 71 off in step 180,such that the main battery 65 stops charging in step 190.

[0028]FIG. 5 illustrates the steps taken by the present invention toensure that the main battery 65 provides power to the unit power andfunctional circuitry 55 if the charger 40 is removed, and also that thebridge battery provides power to the unit power and functional circuitry55 if the charger 40 and the main battery 65 is removed. Beginning instep 230, the charger 40 is powered off. In step 240, the charger diode51 becomes inactive and the FET switch 71 turns off. This causes avoltage drop across the main battery diode 75, causing the main batterydiode 75 to begin conducting and become active in step 250. Since thevoltage level provided to the power bus 90 from the main battery 65 ishigher than the bridge battery voltage 95, the bridge battery 60 doesnot conduct. The main battery then provides power to the power bus 90,so that the portable device can run. The portable device 10 continues tobe powered by main battery 65, until the main battery 65 is disconnectedform the portable device in step 270. Once the main battery 65 isdisconnected, the main battery diode 75 becomes inactive and the bridgebattery diode 61 becomes active in step 280. This is because the voltagelevel on the power bus 90 falls below the voltage level of the bridgebattery 95. In step 290, the device 10 enters the low power mode orsuspend state, and the bridge battery 95 powers the device 10.

[0029]FIG. 6 illustrates the steps taken by the present invention toinsure that the main battery 65 provides the proper level of voltage topower the unit power and functional circuitry 55, if the charger 40 isremoved. Beginning in step 330, the charger 40 is powered off. In step340, the charger diode 50 becomes inactive and the FET switch 71 turnsoff. This causes a voltage drop across the main battery diode 75,causing the main battery diode 75 to begin conducting and become activein step 350. Since the voltage level provided to the power bus 90 fromthe main battery 65 is higher than the bridge battery voltage 95, thebridge battery diode 61 does not conduct. The main battery 65 thenprovides power to the power bus 90, so that the portable device can runin step 360. The comparator 85 monitors the voltage level of the powerbus 90 to insure that it stays above a predetermined level in step 370.If the voltage is low on the power bus 90, the comparator 85 transmitsan output low main battery signal, in step 380, to the device power andfunctional circuitry 55, so that the device 10 can enter the low powermode in step 390. It should be appreciated that the signal may becoupled to an LED or a display to provide a visual warning to the userof device 10. It should also be appreciated that the device 10 couldpower down upon detection of a low battery voltage level.

[0030] The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications alterations, and equivalents thereof.

What is claimed is:
 1. A power distribution system for a providing powerto functional and power circuitry on a portable device, comprising; amain battery system coupled to a power bus through a first switchsystem; and a charging system coupleable to the power bus through asecond switch system; wherein the charging system provides power to thepower bus through the second switch system while the charging system iscoupled to the second switch system and the main battery system providespower to the power bus through the first switch system upon removal ofthe charging system.
 2. The system of claim 1 , further including abridge battery system coupled to the power bus through a third switchsystem wherein the voltage level of the bridge battery is lower than thevoltage level of the main battery and removal of the main battery systemand the charging system causes the third switch system to allow thebridge battery system to provide power to the power bus.
 3. The systemof claim 1 , wherein the second switch system includes a switch disposedbetween the main battery system and the power bus, the charging systemcharging the battery system when the switch is activated and notcharging the battery system when the switch is deactivated.
 4. Thesystem of claim 3 , wherein the second switch system includes a mainbattery diode and the switch coupled in parallel with the main batterydiode.
 5. The system of claim 4 , wherein the switch is a FET switch. 6.The system of claim 3 , further including a charge status monitoringsystem, the charge status monitoring system monitoring the charge levelof the main battery wherein the charging of the main battery isdiscontinued once the battery meets a predetermined charge leveldetermined by the charge status monitoring system.
 7. The system ofclaim 6 , wherein the charge status monitor system includes a timerdevice that causes the charging of the main battery to discontinue aftera predetermined amount of time.
 8. The system of claim 7 , wherein thecharge status monitor system includes a current measuring device formonitoring the current draw from the charger.
 9. The system of claim 3 ,further including a low main battery power monitoring system wherein thelow main battery power monitoring system detects a voltage level below apredetermined level of the main battery and outputs a low main batterysignal.
 10. The system of claim 9 , wherein the low main battery powermonitoring system includes a comparator circuit coupled at a firstterminal to the power bus and coupled to a second terminal by areference signal.
 11. The system of claim 1 , wherein the first switchsystem is a charger diode.
 12. The system of claim 1 , wherein thesecond switch system is a main battery diode.
 13. A power distributionsystem for a providing power to functional and power circuitry on aportable device, comprising; a main battery system coupled to a powerbus through a first switch system; and a bridge battery system coupledto the power bus through a second switch system wherein the voltagelevel of the bridge battery is lower than the voltage level of the mainbattery and removal of the main battery system causes the second switchsystem to allow the bridge battery system to provide power to the powerbus.
 14. The system of claim 13 , wherein the first switch systemincludes a main battery diode.
 15. The system of claim 13 , wherein thesecond switch system is a bridge battery diode.
 16. The system of claim13 , further including a charging system coupled to the portable deviceand the power bus through a third switch system wherein the chargingsystem has a voltage level that is at a predetermined level above avoltage level of the main battery system and the charging systemprovides power to the power bus through the third switch system when thecharger is operating.
 17. The system of claim 16 , wherein the secondswitch system is a charger diode.
 18. The system of claim 13 , whereinthe first switch system includes a switch disposed between the mainbattery system and the power bus, the charging system charging thebattery system when the switch is activated and not charging the batterysystem when the switch is deactivated.
 19. The system of claim 18 ,wherein the first switch system includes a main battery diode and theswitch coupled in parallel with the main battery diode.
 20. The systemof claim 19 , wherein the switch is a FET switch.
 21. The system ofclaim 13 , further including a charge status monitoring system, thecharge status monitoring system monitoring the charge level of the mainbattery wherein the charging of the main battery is discontinued oncethe battery meets a predetermined charge level determined by the chargestatus monitoring system.
 22. The system of claim 21 , wherein thecharge status monitor system includes a timer device that causes thecharging of the main battery to discontinue after a predetermined amountof time.
 23. The system of claim 21 , wherein the charge status monitorsystem includes a current measuring device for monitoring the currentdraw from the charger.
 24. The system of claim 13 , further including alow main battery power monitoring system wherein the low main batterypower monitoring system detects a voltage level below a predeterminedlevel of the main battery and outputs a low main battery signal.
 25. Thesystem of claim 24 , wherein the low main battery power monitoringsystem includes a comparator circuit coupled at a first terminal to thepower bus and coupled to a second terminal by a reference signal.
 26. Amethod for power distribution in a portable device including a power buscoupled to a device power and functional circuitry, comprising the stepsof; providing a main battery system coupled to a power bus through amain battery diode; providing a charging system coupled to the power busthrough a charger diode; and providing a bridge battery system coupledto the power bus through a bridge battery diode wherein the chargingsystem has a voltage level above the main battery voltage level and themain battery has a voltage level above the bridge battery, such that thecharger powers the power bus if the charger is present, the main batterypowers the power bus if the charger is not present and the bridgebattery powers the bus if the charger and the main battery are notpresent.
 27. The method of claim 26 , further including the step ofrecharging the main battery system from the charging system.
 28. Themethod of claim 26 , further including the step of monitoring the chargelevel of the main battery and discontinuing the charging of the mainbattery upon reaching a predetermined charge level.
 29. The method ofclaim 26 , further including the step of monitoring the voltage level ofthe main battery and outputting a low battery voltage signal when thevoltage level falls below a predetermined level.
 30. A powerdistribution system for a providing power to functional and powercircuitry on a portable device, comprising; means for providing mainbattery power, the means for providing main battery power includingmeans for coupling the means for providing main battery power to a powerbus; and means for recharging the means for providing main batterypower, the means for recharging the means for providing main batterypower including means for coupling the means for recharging to a powerbus; wherein the means for charging has a voltage level that is at apredetermined level above a voltage level of the means for providingmain battery power causing the means for charging to simultaneouslyprovide power to the power bus and recharge the means for providing mainbattery power.
 31. The system of claim 30 , further including means forproviding power to the power bus in the event the means for providingmain battery power is removed, the means for providing power to thepower bus in the event the means for providing main battery power isremoved including means for coupling the means for providing power tothe power bus in the event the means for providing main battery power isremoved to the power bus
 32. The system of claim 30 , further includingmeans for monitoring the charge level of the means for providing mainbattery power.
 33. The system of claim 30 , further including means formonitoring the voltage level of the means for providing main batterypower.
 34. The system of claim 30 , further including means for enablingand disabling the recharging of the means for providing main batterypower.
 35. A power distribution system for a providing power tofunctional and power circuitry on a portable device, comprising; a mainbattery system coupled to a power bus through a main battery diode; acharging system coupled to the power bus through a charger diode; and abridge battery system coupled to the power bus through a bridge batterydiode; wherein the charging system has a voltage level that is at apredetermined level above a voltage level of the main battery system andthe voltage level of the bridge battery system is lower than the voltagelevel of the main battery system, such that if the charging system ispresent the charger diode is forward biased and the charging systemprovides power to the power bus, if the charging system is not presentthe main battery diode becomes forward biased and the main batterysystem provides power to the power bus and if the charging system andthe main battery system are not present the bridge battery diode becomesforward biased and the bridge battery system provides power to the powerbus.